Lung volume measurement is useful in long-term intensive care to optimize lung therapy and ventilation for the pulmonary characteristics of a patient. In carrying out such therapies, the patient is often sedated, lying for long periods in an unchanged posture, and sometimes ventilated with a high oxygen concentration gas mixture. Under such conditions, the lungs tend to collapse and smaller airways gradually close at the end of expiration. Infiltrates from pulmonary perfusion may exacerbate these conditions. This gradual collapse and closure is called alveolar de-recruitment. If allowed to progress far enough, this will cause insufficient gas exchange in the lungs that may be observed, e.g. by arterial oxygen monitoring. Such de-recruitment can be reversed by periodic recruitment in which the lung is opened by providing elevated pressure breathing gases to the lungs.
However, long before the deterioration of gas exchange in the lungs becomes apparent, de-recruitment may gradually damage the lung during long-term ventilation. The mechanism behind this damage is a shearing of alveolar tissue against itself caused by cyclic opening and closing during tidal breathing. Much intensive care equipment currently lacks accessible bedside methods to observe such de-recruitment and loss of lung volume occurring in the lungs.
Methods to obtain a lung volume measurement at the bedside typically use an inert gas wash out technique. In this technique, a gas that is metabolically inert with respect to gas exchange in the lungs is provided in the inhaled breathing gases. The most common inert gases used for this purpose are sulphur hexafluoride (SF6) and nitrogen (N2), the latter providing advantage of already having existing concentration control in ventilator equipments since nitrogen is a component of atmospheric air. It is also possible to use an inert gas wash in technique to measure FRC.
In a state of equilibrium, the amount of inhaled inert gas equals with the amount of exhaled inert gas. To commence a lung volume measurement, the inert gas concentration in inhaled breathing gases is changed. The concentration of inert gas in the lungs is driven toward a new equilibrium. In the process of attaining the new equilibrium, a net amount of inert gas is exchanged in the lungs through breathing. Measuring the amount of this exchange and relating that with the respective change in lung inert gas concentration gives an indication of end-expiratory lung volume, namely the functional residual capacity (FRC) of the lungs. The functional residual capacity of the lungs is the volume of the lungs remaining after a normal exhalation. The FRC volume of the lungs is presented in Equation 1 as
                              FRC          n                =                                                                      ∑                                  breaths                  =                  1                                n                            ⁢                              Δ                ⁢                                                                  ⁢                                  V                                      N                    ⁢                                                                                  ⁢                    2                                                                                                      ETN                2                baseline                            -                              ETN                2                n                                                                                  (        1        )            In Equation 1, the numerator is the volume of exhaled inert gas, such as nitrogen (N2), and the denominator is the difference or change in expired end tidal nitrogen concentrations occurring during the measurement commencing from a baseline amount.
Using Equation 1, a new FRC value is determined for each breath subsequent to the change in inspired inert gas concentration. These values are not constant but tend to increase with successive breaths as the measurement moves to a termination. Criteria for concluding the measurement to establish an FRC value are therefore necessary in order to achieve the reproducibility of the FRC measurement for comparative and other purposes. Algorithms have been presented for this purpose. See, for example, U.S. Published Patent Application No. 2002/0052560 that describes a method to stop the measurement when FRC convergence to an end value appears to develop. Such convergence is defined as occurring when the FRC for a preset number of successive breaths is within a preset tolerance range. As an example, the number of breaths is presented as three and the tolerance as 5%-20% of the last calculated FRC value, with the last calculated value representing the final result for the FRC. As alternative concluding criteria, the measurement resolution of the gas concentration analyzer used in the measurement of FRC is presented. Both of these methods however lack a physiological connection with the primary function of the lung, the provision of oxygen to the blood and the removal of carbon dioxide from the blood.